Sounds like my DD, she is 5 in March.
Mind of her own all of a sudden and an attitude to match, likes to think she can do what ever she wants when she wants.
Sometimes I have to get a bit tough on her, restricting some things I normally wouldn't, usually I am pretty soft, but lately I've had to be pretty stern with her most of the time.

I think she knows big change is coming and is grasping for attention any way she can, because I am not able to do as much with her as I normally would, and it's only gonna get worse when bubs comes.

Have you tried some one on one special time with him, just the two of you.
I am planning on doing it as much as I can when bub is born, been just the two of us for such a long time, it's a big adjustment for them.
She helps me with "special" jobs and gets rewarded.

I've also found ignoring the small stuff works well, not giving negative attention all the time, praising a lot when she does something good and not letting her get away with as much (like attituse, lying, not tidying up etc), all in combination seem to be keeping her well behaved most of the time.

Not sure how much sense I am making lol, she is bugging me for lollies as I sit here typing (she has no chance hehe)